If you’re preparing for the United States Medical Licensing Examination® (USMLE®) Step 2 exam, you might want to know which questions are most often missed by test-prep takers. Check out this example from Kaplan Medical, and read an expert explanation of the answer. Also check out all posts in this series.  

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A 17-year-old girl is brought to the emergency department because of a two-day history of a painful and swollen right ankle. She also has had a fever for two days, with temperatures up to 38.9 ºC (102 ºF). She plays rugby after school but does not recall any recent trauma to the ankle or recent illness. Her parents decided to bring her to the hospital tonight because she has been unable to sleep due to the pain. Her temperature is 38.8 ºC (101.8 ºF).

On physical examination, her right ankle is markedly swollen, brightly erythematous and very painful to touch. She is unwilling to move the ankle or put weight on it. Aspirated fluid under ultrasound guidance from the joint shows a white blood count of 55,000/mm3 with increased protein and decreased glucose.

Which of the following would be diagnostic of this condition?

A. Aspirated joint fluid showing deposits of calcium pyrophosphate crystals.

B. Aspirated joint fluid showing deposits of monosodium urate crystals.

C. Aspirated joint fluid showing gram-positive cocci in clusters.

D. Blood for rheumatoid factor and anti-cyclic citrullinated peptide positive.

E. Blood for Western immunoblotting showing five of 10 IgG bands and two of three IgG bands positive for B. burgdorferi.

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The correct answer is C.

This otherwise healthy patient with acute onset of fever, pain in one joint and findings of inflammation at the joint most likely has septic arthritis. Additionally, given her possible exposure to minor S. aureus infections from rugby practice, and her joint fluid showing elevated white blood cell counts and low glucose, it is expected to discover gram-positive cocci in clusters in her aspirated joint fluid.

Choices A and B: Pseudogout refers to the deposition of calcium pyrophosphate crystals in the joint and most commonly affects the knee. Gout is characterized by serum uric acid elevation and urate (monophosphate urate crystals) deposition in the joint and soft tissues, and most commonly affects the first metatarsophalangeal joint. These are both clinically indistinguishable from each other, are much less common in children and are not associated with high fevers.

Choice D: Positive rheumatoid factor and positive anti-cyclic citrullinated peptide is indicative of polyarticular juvenile idiopathic arthritis. This is characterized by onset of symptoms before 16 years of age and with symptoms lasting at least six weeks. It is a chronic synovitis with or without systemic inflammatory manifestations. The etiology is unknown, and it is typically a diagnosis of exclusion.

Choice E: Immunoblotting showing five of 10 IgG bands and two of three IgG bands positive for B. burgdorferi is indicative of Lyme disease. This is a tick-borne illness that initially presents with nonspecific constitutional symptoms and the classic erythema migrans. Arthritis is a feature occurring in the late stage of Lyme disease and classically involves the knee or other large joints.

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  • Septic arthritis is most commonly attributable to hematogenous spread. It most often presents in the knee, hip or ankle joint, but may involve any joint. In infants there can be multiple joint involvement.
  • The overall most common organism is S. aureus. Others include group A streptococcus, pneumococcus, and Kingella kingae. H. influenzae type B is no longer a common causative organism. Patients with hemolytic anemias are at risk for infection with Salmonella.
  • Arthrocentesis shows synovial fluid that is usually purulent with a neutrophil count greater than 50,000/mm3 and with high protein and low glucose concentration. The diagnosis is confirmed with a positive Gram stain or culture.

For more prep questions on USMLE Steps 1, 2 and 3, view other posts in this series.

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